Heretofore, a naphtha steam reforming process and a partial oxidizing process of residual oil, etc., have been known as processes for producing gas low in methane content. However, the former utilizes materials of high cost and cannot utilize a poor quality fuel such as cokes; and the latter has disadvantages in that it has an inherent danger due to using an oxidizing agent such as oxygen, and materials extensively rich in carbon content cannot be utilized.